Brecciation, alteration and mineralization in the Central Tennessee zinc district

Abstract

Major zinc deposits have recently been discovered in stratabound breccias in the Mascot Dolomite of the upper Knox Group in Central Tennessee and southern Kentucky. The Mascot Dolomite is a Lower Ordovician sequence of very finely crystalline dolostone units with interbedded limestones and coarsely crystalline dolomitized equivalents of these limestones. The breccias are similar to those of the well-known zinc deposits within the Knox Group of East Tennessee but with some distinct differences. The dominant minerals in Central Tennessee are reddish brown sphalerite, barite, fluorite, galena, and calcite. In East Tennessee pale yellow sphalerite and dolomite are the only major minerals cementing the breccias. Sulfide mineral deposition in Central Tennessee was significantly later than breccia formation and may have been as late as late Paleozoic. In East Tennessee sulfide mineralization took place in temporal proximity to limestone dissolution and dolomitization and occurred during the early Middle Ordovician.In Central Tennessee sulfide-depositing solutions were in equilibrium with calcite and could not have been responsible for dissolution and subsequent collapse of overlying dolostone beds to form ore-hosting breccias. Stratigraphic relationships suggest that secondary dolomitization and dissolution were related processes and that the dominance of limestone dissolution in preference to volume-for-volume dolomitization controlled the development of the breccia systems. Precipitation of coarse crystals of ore and gangue minerals may have resulted from slow mixing of separate fluids containing reduced sulfur and metals or by gradual reduction of sulfate present in a metal-bearing brine.